A009723 Expansion of tan(tanh(x)/cos(x)).

1, 3, 57, 1995, 128881, 12775059, 1796958633, 342118793307, 84276974508769, 26141579246291235, 9956883572644073241, 4570240365824839488747, 2487549038143714062879313, 1584207614996350912703688627

Offset: 0

Author

R. H. Hardin

Keywords

• nonn

Programs

Mathematica

With[{nn=30},Take[CoefficientList[Series[Tan[Tanh[x]/Cos[x]],{x,0,nn}],x] Range[0,nn-1]!,{2,-1,2}]] (* Harvey P. Dale, Dec 15 2014 *)

Formula

a(n) ~ c * d^n * n^(2*n + 3/2), where d = 0.491131788453837761770391039276... and c = 3.6558020994214430118438069889... - Vaclav Kotesovec, Nov 27 2024

Extensions

Extended and signs tested Mar 15 1997 by Olivier Gérard.

Prior Mathematica program replaced by Harvey P. Dale, Dec 15 2014